In large-capacity electric power conversion devices, the converter output is high voltage or large current, and therefore, many large-capacity electric power conversion devices are configured with a plurality of converters multiplexed in series or parallel. It is known that multiplexing converters not only increases the converter capacity, but also reduces harmonics contained in an output voltage waveform by synthesizing outputs, thus reducing harmonic current flowing to outside of the conversion device.
There are various methods for multiplexing a converter: reactor multiplexing, transformer multiplexing, direct multiplexing, etc. In the case of transformer multiplexing, since an AC side is isolated by transformers, there is an advantage that common direct current can be used among the transformers. However, there is a disadvantage that, in the case where output voltage is high, the configuration of the multiplexed transformer is complicated and the cost of the transformer increases (for example, Patent Document 1).
Considering the above, as an electric power conversion device that is suitable for high-voltage usage and does not require a multiplexed transformer, a multilevel converter is proposed in which outputs of a plurality of converters are connected in cascade. One example of such multilevel converters is a modular multilevel converter (hereinafter, referred to as an MMC). Since the MMC can be configured to have high withstand voltage and a large capacity, the MMC is a converter that can be interconnected to a power grid, and a wide range of applications thereof is being considered, e.g., high-voltage DC (HVDC) power transmission, BTB (Back To Back) (asynchronous interconnection device), a frequency conversion (FC) device, and a static synchronous compensator (STATCOM).
The MMC is composed of an arm in which a plurality of unit converters called cells (hereinafter, referred to as unit cells) are connected in cascade. Each unit cell includes a plurality of semiconductor switches and a DC capacitor, and through ON/OFF control of the semiconductor switches, outputs the voltage across the DC capacitor or zero voltage.
A three-phase MMC can have various configurations depending on a connection manner for arms. One of such converter configurations is a delta-connection cascade configuration. A delta-connection cascade converter has arms which are delta-connected and each of which includes a plurality of unit cells connected in cascade, to which a reactor is further connected in series, and the delta-connection cascade converter is connected in parallel to an AC power grid via a reactor or a transformer. Thus, two current components exist: current flowing between the grid and each phase, and current circulating within the delta-connection circuitry of the converter without being outputted to the grid side. Therefore, in the three-phase MMC, it is necessary to control these current components. A DC capacitor is provided in each cell, and the DC capacitor does not have a power supply. Therefore, it is necessary to control the voltage of the DC capacitor within a certain range.
In the three-phase MMC, it is necessary to control a plurality of current components and DC capacitor voltage. A method for controlling imbalance of DC capacitor voltages among phases due to grid imbalance by using circulating current is disclosed (for example, Patent Document 1).